Ram completes 60% of the task in 15 days.
i.e., he completes 4% of the task in a day.

Rahim is 50% as efficient as Ram is.
Therefore, Rahim will complete 2% of the task in a day.

Rachel is 50% as efficient as Rahim is.
Therefore, Rachel will complete 1% of the task in a day.

Together, Ram, Rahim and Rachel will complete 4 + 2 + 1 = 7% of the work in a day.
Ram, working alone, had already completed 60% of the task.
They have another 40% of the task to be completed.
Therefore, they will take 407407 or 55757 more days to complete the task.

   The correct answer is Choice (3).